LM25069PMM-2

LM25069 Positive Low Voltage Power Limiting Hot Swap Controller July 15, 2009 LM25069 Positive Low Voltage Power Limiting Hot Swap Controller General Description The LM25069 positive hot swap controller provides intelligent control of the power supply voltage to the load during insertion and removal of circuit cards from a live system backplane or other "hot" power sources. The LM25069 provides in-rush current control to limit system voltage droop and transients. The current limit and power dissipation in the external series pass N-Channel MOSFET are programmable, ensuring operation within the Safe Operating Area (SOA). The POWER GOOD output indicates when the output voltage is within 1.3V of the input voltage. The input under-voltage and over-voltage lockout levels and hysteresis are programmable, as well as the initial insertion delay time and fault detection time. The LM25069-1 latches off after a fault detection, while the LM25069-2 automatically restarts at a fixed duty cycle. The LM25069 is available in a 10 pin MSOP package. Features ■ Operating range: +2.9V to +17V ■ In-rush current limit for safe board insertion into live power ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ sources Programmable maximum power dissipation in the external pass device Adjustable current limit Circuit breaker function for severe over-current events Internal high side charge pump and gate driver for external N-channel MOSFET Adjustable under-voltage lockout (UVLO) and hysteresis Adjustable over-voltage lockout (OVLO) and hysteresis Initial insertion timer allows ringing and transients to subside after system connection Programmable fault timer avoids nuisance trips Active high open drain POWER GOOD output Available in latched fault and automatic restart versions Applications ■ Server Backplane Systems ■ Base Station Power Distribution Systems ■ Solid State Circuit Breaker Package ■ MSOP-10 Typical Application 30086701 Positive Power Supply Control © 2009 National Semiconductor Corporation 300867 www.national.com LM25069 Connection Diagram 30086702 Top View 10-Lead MSOP Ordering Information Order Number LM25069PMME-1 LM25069PMM-1 LM25069PMMX-1 LM25069PMME-2 LM25069PMM-2 LM25069PMMX-2 Fault Response Latch Off Latch Off Latch Off Auto Restart Auto Restart Auto Restart MSOP-10 MUB10A Package Type NSC Package Drawing Supplied As 250 Units on Tape and Reel 1000 Units on Tape and Reel 3500 Units on Tape and Reel 250 Units on Tape and Reel 1000 Units on Tape and Reel 3500 Units on Tape and Reel Pin Descriptions Pin # 1 Name SENSE Description Current sense input Applications Information The voltage across the current sense resistor (RS) is measured from VIN to this pin. If the voltage across RS reaches 50mV the load current is limited and the fault timer activates. A small ceramic bypass capacitor close to this pin is recommended to suppress transients which occur when the load current is switched off. An external resistor divider from the system input voltage sets the under-voltage turn-on threshold. An internal 20 µA current source provides hysteresis. The enable threshold at the pin is 1.17V. This pin can also be used for remote shutdown control. An external resistor divider from the system input voltage sets the over-voltage turn-off threshold. An internal 20 µA current source provides hysteresis. The disable threshold at the pin is 1.16V. An external capacitor connected to this pin sets the insertion time delay and the Fault Timeout Period. The capacitor also sets the restart timing of the LM25069-2. An external resistor connected to this pin, in conjunction with the current sense resistor (RS), sets the maximum power dissipation allowed in the external series pass MOSFET. An open drain output. When the external MOSFET VDS decreases below 1.3V, the PGD indicator is active (high). When the external MOSFET VDS increases above 1.9V the PGD indicator switches low. 2 3 VIN UVLO/EN Positive supply input Under-voltage lockout 4 OVLO Over-voltage lockout 5 6 7 GND TIMER PWR Circuit ground Timing capacitor Power limit set 8 PGD Power Good indicator www.national.com 2 LM25069 Pin # 9 10 Name OUT GATE Description Output feedback Gate drive output Applications Information Connect to the output rail (external MOSFET source). Internally used to determine the MOSFET VDS voltage for power limiting, and to control the PGD indicator. Connect to the external MOSFET’s gate. This pin's voltage is limited at 19.5V above ground. 3 www.national.com LM25069 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to GND (Note 5) SENSE, OUT, PGD to GND UVLO to GND OVLO to GND VIN to SENSE ESD Rating (Note 2) Human Body Model -0.3V to 20V -0.3V to 20V -0.3V to 20V -0.3V to 20V -0.3V to +0.3V 2kV Storage Temperature Junction Temperature Lead Temperature (soldering 4 sec) -65°C to +150°C +150°C +260°C Operating Ratings VIN Supply Voltage PGD Off Voltage Junction Temp. Range +2.9V to 17V 0V to 17V −40°C to +85°C Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of -40°C to +85°C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless otherwise stated the following conditions apply: VIN = 12V. Symbol Parameter Input Current, enabled Input Current, disabled Power On Reset threshold at VIN POR hysteresis OUT bias current, enabled OUT bias current, disabled (Note 3) UVLO threshold UVLO hysteresis current UVLO delay UVLO bias current OVLO threshold OVLO hysteresis current OVLO delay OVLO bias current OVLO = 2V Delay to GATE high Delay to GATE low OVLOBIAS PWRLIM-1 PWRLIM-2 IPWR RSAT(PWR) IGATE PWR pin current PWR pin impedance when disabled Source current Sink current OVLO = 1V 19 19 25 25 -15 140 -27 1.5 160 18 -20 2 260 19.5 -13 2.5 375 20.5 UVLO = 1V Delay to GATE high Delay to GATE low UVLOBIAS OVLOTH OVLOHYS OVLODEL UVLO = 3V 1.142 -26 1.16 -20 16 8.2 1 31 31 µA mV mV µA Ω µA mA mA V Conditions UVLO = 2V and OVLO = 0.7V, VIN = 14V UVLO = 0.7V or OVLO = 2V VIN Increasing VIN decreasing OUT = VIN, Normal operation Disabled, OUT = 0V, SENSE = VIN 1.154 15 Min. Typ. 1.6 1.0 2.6 150 0.30 -12 1.17 20 15 8.3 1 1.185 -15 µA V µA µs 1.183 26 V µA µs Max. 2.4 1.6 2.8 Units mA mA V mV µA Input (VIN pin) IIN-EN IIN-DIS POR PORHYS OUT pin IOUT-EN IOUT-DIS UVLOTH UVLOHYS UVLODEL UVLO, OVLO pins Power Limit (PWR pin) Power limit sense voltage (VIN-SENSE) SENSE-OUT = 12V, RPWR = 69.8 kΩ SENSE-OUT = 6V, RPWR = 34.8 kΩ VPWR = 2.5V UVLO = 0.7V Normal Operation UVLO = 1V VIN - SENSE = 150 mV or VIN < POR, VGATE = 5V VGATE Gate output voltage in normal operation GATE voltage with respect to ground Gate Control (GATE pin) www.national.com 4 LM25069 Symbol Current Limit VCL tCL ISENSE Parameter Threshold voltage Response time SENSE input current Conditions VIN-SENSE voltage VIN-SENSE stepped from 0 mV to 80 mV Enabled, SENSE = OUT Disabled, OUT = 0V Enabled, OUT = 0V Min. 45 Typ. 50 15 23 12 62 Max. 55 Units mV µs µA Circuit Breaker VCB tCB Threshold voltage Response time VIN - SENSE VIN - SENSE stepped from 0 mV to 150 mV, time to GATE low, no load 1.6 Restart cycles (LM25069-2) End of 8th cycle (LM25069-2) Re-enable Threshold (LM25069-1) ITIMER Insertion time current Sink current, end of insertion time Fault detection current Fault sink current DCFAULT tFAULT PGDTH PGDVOL PGDIOH Fault Restart Duty Cycle Fault to GATE low delay Threshold measured at SENSE-OUT Output low voltage Off leakage current LM25069-2 only TIMER pin reaches the upper threshold Decreasing Threshold Hysteresis ISINK = 2 mA VPGD = 17V TIMER pin = 2V -7.5 1.5 -110 1.6 0.9 75 95 0.19 110 0.36 mV µs Timer (TIMER pin) VTMRH VTMRL Upper threshold Lower threshold 1.72 1.0 0.3 0.3 -5.5 2 -80 2.5 0.67 20 1.3 0.6 15 30 1 mV µA 1.9 -3.5 2.5 -50 3.4 1.85 1.1 V V V V µA mA µA µA % µs V Power Good (PGD pin) Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and conditions see the Electrical Characteristics. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 3: OUT bias current (disabled) due to leakage current through an internal 1.0 MΩ resistance from SENSE to VOUT. Note 4: For detailed information on soldering plastic MSOP packages refer to the Packaging Databook available from National Semiconductor Corporation. Note 5: Current out of a pin is indicated as a negative number. 5 www.national.com LM25069 Typical Performance Characteristics 25°C, VIN = 12V VIN Pin Input Current vs. VIN Unless otherwise specified the following conditions apply: TJ = SENSE Pin Input Current 30086703 30086704 OUT Pin Input Current GATE Pin Voltage 30086705 30086706 GATE Pin Source Current MOSFET Power Dissipation Limit 30086707 30086709 www.national.com 6 LM25069 PGD Pin Low Voltage vs. Sink Current Input Current, Enabled vs. Temperature 30086708 30086758 UVLO Threshold vs. Temperature UVLO Hysteresis Current vs. Temperature 30086757 30086755 OVLO Threshold vs. Temperature OVLO Hysteresis Current vs. Temperature 30086766 30086756 7 www.national.com LM25069 Current Limit Threshold vs. Temperature Circuit Breaker Threshold vs. Temperature 30086759 30086760 Power Limit Threshold vs. Temperature GATE Output Voltage vs. Temperature 30086761 30086762 GATE Source Current vs. Temperature PGD Low Voltage vs. Temperature 30086763 30086765 www.national.com 8 LM25069 Block Diagram 30086710 30086711 FIGURE 1. Basic Application Circuit 9 www.national.com LM25069 Functional Description The LM25069 is designed to control the in-rush current to the load upon insertion of a circuit card into a live backplane or other "hot" power source, thereby limiting the voltage sag on the backplane’s supply voltage, and the dV/dt of the voltage applied to the load. Effects on other circuits in the system are minimized, preventing possible unintended resets. A controlled shutdown when the circuit card is removed can also be implemented using the LM25069. In addition to a programmable current limit, the LM25069 monitors and limits the maximum power dissipation in the series pass device to maintain operation within the device Safe Operating Area (SOA). Either current limiting or power limiting for an extended period of time results in the shutdown of the series pass device. In this event, the LM25069-1 latches off until the circuit is reenabled by external control, while the LM25069-2 automatically restarts with defined timing. The circuit breaker function quickly switches off the series pass device upon detection of a severe over-current condition. The Power Good (PGD) output pin indicates when the output voltage is within 1.3V of the system input voltage (VSYS). Programmable under-voltage lock-out (UVLO) and over-voltage lock-out (OVLO) circuits enable the LM25069 when the system input voltage is between the desired thresholds. The typical configuration of a circuit card with LM25069 hot swap protection is shown in Figure 2. 30086712 FIGURE 2. LM25069 Application Power Up Sequence The VIN operating range of the LM25069 is +2.9V to +17V, with a transient capability to 20V. Referring to the Block Diagram and Figure 1 and Figure 3, as the voltage at VIN initially increases, the external N-channel MOSFET (Q1) is held off by an internal 260 mA pull-down current at the GATE pin. The strong pull-down current at the GATE pin prevents an inadvertent turn-on as the MOSFET’s gate-to-drain (Miller) capacitance is charged. Additionally, the TIMER pin is initially held at ground. When the VIN voltage reaches the POR threshold the insertion time begins. During the insertion time, the capacitor at the TIMER pin (CT) is charged by a 5.5 µA current source, and Q1 is held off by a 2 mA pull-down current at the GATE pin regardless of the VIN voltage. The insertion time delay allows ringing and transients at VIN to settle before Q1 is enabled. The insertion time ends when the TIMER pin voltage reaches 1.72V. CT is then quickly discharged by an internal 2 mA pull-down current. The GATE pin then switches on Q1 when VSYS exceeds the UVLO threshold. If VSYS is above the UVLO threshold at the end of the insertion time, Q1 switches on at that time. The GATE pin charge pump sources 20 µA to charge Q1’s gate capacitance. The maximum voltage at the GATE pin is limited by an internal 19.5V zener diode. As the voltage at the OUT pin increases, the LM25069 monitors the drain current and power dissipation of MOSFET Q1. In-rush current limiting and/or power limiting circuits actively control the current delivered to the load. During the in-rush limiting interval (t2 in Figure 3) an internal 80 µA fault timer current source charges CT. If Q1’s power dissipation and the input current reduce below their respective limiting thresholds before the TIMER pin reaches 1.72V the 80 µA current source is switched off, and CT is discharged by the internal 2.5 µA current sink (t3 in Figure 3). The in-rush limiting interval is complete when the voltage at the OUT pin increases to within 1.3V of the input voltage (VSYS), and the PGD pin switches high. If the TIMER pin voltage reaches 1.72V before in-rush current limiting or power limiting ceases (during t2), a fault is declared and Q1 is turned off. See the Fault Timer & Restart section for a complete description of the fault mode. www.national.com 10 LM25069 30086713 FIGURE 3. Power Up Sequence (Current Limit only) Gate Control A charge pump provides the voltage at the GATE pin to enhance the N-Channel MOSFET’s gate. During normal operating conditions (t3 in Figure 3) the gate of Q1 is held charged by an internal 20 µA current source. The voltage at the GATE pin (with respect to ground) is limited by an internal 19.5V zener diode. See the graph “GATE Pin voltage. Since the gate-to-source voltage applied to Q1 could be as high as 19.5V during various conditions, a zener diode with the appropriate voltage rating must be added between the GATE and OUT pins if the maximum VGS rating of the selected MOSFET is less than 19.5V. The external zener diode must have a forward current rating of at least 260 mA. When the system voltage is initially applied, the GATE pin is held low by a 260 mA pull-down current. This helps prevent an inadvertent turn-on of the MOSFET through its drain-gate capacitance as the applied system voltage increases. During the insertion time (t1 in Figure 3) the GATE pin is held low by a 2 mA pull-down current. This maintains Q1 in the offstate until the end of t1, regardless of the voltage at VIN or UVLO. Following the insertion time, during t2 in Figure 3, the gate voltage of Q1 is modulated to keep the current or power dissipation level from exceeding the programmed levels. While in the current or power limiting mode the TIMER pin capacitor is charging. If the current and power limiting cease before the TIMER pin reaches 1.72V the TIMER pin capacitor then discharges, and the circuit enters normal operation. If the in-rush limiting condition persists such that the TIMER pin reached 1.72V during t2, the GATE pin is then pulled low by the 2 mA pull-down current. The GATE pin is then held low until either a power up sequence is initiated (LM25069-1), or until the end of the restart sequence (LM25069-2). See the Fault Timer & Restart section. If the system input voltage falls below the UVLO threshold, or rises above the OVLO threshold, the GATE pin is pulled low by the 2 mA pull-down current to switch off Q1. Current Limit The current limit threshold is reached when the voltage across the sense resistor RS (VIN to SENSE) reaches 50 mV. In the current limiting condition, the GATE voltage is controlled to limit the current in MOSFET Q1. While the current limit circuit is active, the fault timer is active as described in the Fault Timer & Restart section. If the load current falls below the current limit threshold before the end of the Fault Timeout Period, the LM25069 resumes normal operation. For proper operation, the RS resistor value should be no larger than 200 11 www.national.com LM25069 mΩ. Higher values may result in instability in the current limit control loop. Fault Timer & Restart When the current limit or power limit threshold is reached during turn-on or as a result of a fault condition, the gate-tosource voltage of Q1 is modulated to regulate the load current and power dissipation in Q1. When either limiting function is activated, an 80 µA fault timer current source charges the external capacitor (CT) at the TIMER pin as shown in Figure 5 (Fault Timeout Period). If the fault condition subsides during the Fault Timeout Period before the TIMER pin reaches 1.72V, the LM25069 returns to the normal operating mode and CT is discharged by the 2.5 µA current sink. If the TIMER pin reaches 1.72V during the Fault Timeout Period, Q1 is switched off by a 2 mA pull-down current at the GATE pin. The subsequent restart procedure then depends on which version of the LM25069 is in use. The LM25069-1 latches the GATE pin low at the end of the Fault Timeout Period. CT is then discharged to ground by the 2.5 µA fault current sink. The GATE pin is held low by the 2 mA pull-down current until a power up sequence is externally initiated by cycling the input voltage (VSYS), or momentarily pulling the UVLO pin below its threshold with an open-collector or open-drain device as shown in Figure 4. The voltage at the TIMER pin must be less than 0.3V for the restart procedure to be effective. Circuit Breaker If the load current increases rapidly (e.g., the load is shortcircuited) the current in the sense resistor (RS) may exceed the current limit threshold before the current limit control loop is able to respond. If the current exceeds approximately twice the current limit threshold (95 mV/RS), Q1 is quickly switched off by the 260 mA pull-down current at the GATE pin, and a Fault Timeout Period begins. When the voltage across RS falls below 95 mV the 260 mA pull-down current at the GATE pin is switched off, and the gate voltage of Q1 is then determined by the current limit or the power limit functions. If the TIMER pin reaches 1.72V before the current limiting or power limiting condition ceases, Q1 is switched off by the 2 mA pulldown current at the GATE pin as described in the Fault Timer & Restart section. Power Limit An important feature of the LM25069 is the MOSFET power limiting. The Power Limit function can be used to maintain the maximum power dissipation of MOSFET Q1 within the device SOA rating. The LM25069 determines the power dissipation in Q1 by monitoring its drain-source voltage (SENSE to OUT), and the drain current through the sense resistor (VIN to SENSE). The product of the current and voltage is compared to the power limit threshold programmed by the resistor at the PWR pin. If the power dissipation reaches the limiting threshold, the GATE voltage is modulated to regulate the current in Q1. While the power limiting circuit is active, the fault timer is active as described in the Fault Timer & Restart section. 30086715 FIGURE 4. Latched Fault Restart Control The LM25069-2 provides an automatic restart sequence which consists of the TIMER pin cycling between 1.72V and 1V seven times after the Fault Timeout Period, as shown in Figure 5. The period of each cycle is determined by the 80 µA charging current, and the 2.5 µA discharge current, and the value of the capacitor CT. When the TIMER pin reaches 0.3V during the eighth high-to-low ramp, the 20 µA current source at the GATE pin turns on Q1. If the fault condition is still present, the Fault Timeout Period and the restart cycle repeat. The Fault Timeout Period during restart cycles is approximately 18% shorter than the initial fault timeout period which initiated the restart cycle. This is due to the fact that the TIMER pin transitions from 0.3V to 1.72V after each restart time, rather than from ground. www.national.com 12 LM25069 30086716 FIGURE 5. Restart Sequence (LM25069-2) Under-Voltage Lock-Out (UVLO) The series pass MOSFET (Q1) is enabled when the input supply voltage (VSYS) is within the operating range defined by the programmable under-voltage lockout (UVLO) and overvoltage lock-out (OVLO) levels. Typically the UVLO level at VSYS is set with a resistor divider (R1-R3) as shown in Figure 1. Refering to the Block Diagram when VSYS is below the UVLO level, the internal 20 µA current source at UVLO is enabled, the current source at OVLO is off, and Q1 is held off by the 2 mA pull-down current at the GATE pin. As VSYS is increased, raising the voltage at UVLO above its threshold the 20 µA current source at UVLO is switched off, increasing the voltage at UVLO, providing hysteresis for this threshold. With the UVLO pin above its threshold, Q1 is switched on by the 20 µA current source at the GATE pin if the insertion time delay has expired. See the Applications Section for a procedure to calculate the values of the threshold setting resistors (R1-R3). The minimum possible UVLO level at VSYS can be set by connecting the UVLO pin to VIN. In this case Q1 is enabled after the insertion time. drain device, as shown in Figure 6. Upon releasing the UVLO pin the LM25069 switches on the load current with in-rush current and power limiting. 30086717 FIGURE 6. Shutdown Control Power Good Pin The Power Good indicator pin (PGD) is connected to the drain of an internal N-channel MOSFET capable of sustaining 17V in the off-state, and transients up to 20V. An external pull-up resistor is required at PGD to an appropriate voltage to indicate the status to downstream circuitry. The off-state voltage at the PGD pin can be higher or lower than the voltages at VIN and OUT. PGD is switched high when the voltage from SENSE to OUT (the external MOSFET’s VDS) decreases below 1.3V. PGD switches low when the MOSFET’s VDS is increased past 1.9V. If the UVLO pin is taken below its threshold or the OVLO pin taken above its threshold, to disable the LM25069, PGD switches low within 10 µs without waiting for the voltage at OUT to fall. The PGD output pin is high when the voltage at VIN is less than 1.6V. Over-Voltage Lock-Out (OVLO) The series pass MOSFET (Q1) is enabled when the input supply voltage (VSYS) is within the operating range defined by the programmable under-voltage lockout (UVLO) and overvoltage lock-out (OVLO) levels. If VSYS raises the OVLO pin voltage above its threshold Q1 is switched off by the 2 mA pull-down current at the GATE pin, denying power to the load. When the OVLO pin is above its threshold, the internal 20 µA current source at OVLO is switched on, raising the voltage at OVLO to provide threshold hysteresis. When VSYS is reduced below the OVLO level Q1 is enabled. See the Applications Section for a procedure to calculate the threshold setting resistor values. Shutdown Control The load current can be remotely switched off by taking the UVLO pin below its threshold with an open collector or open 13 www.national.com LM25069 Application Information (Refer to Figure 1) CURRENT LIMIT, RS The LM25069 monitors the current in the external MOSFET (Q1) by measuring the voltage across the sense resistor (RS), connected from VIN to SENSE. The required resistor value is calculated from: (1) where ILIM is the desired current limit threshold. If the voltage across RS reaches 50 mV, the current limit circuit modulates the gate of Q1 to regulate the current at ILIM. While the current limiting circuit is active, the fault timer is active as described in the Fault Timer & Restart section. For proper operation, RS must be no larger than 200 mΩ. While the maximum load current in normal operation can be used to determine the required power rating for resistor RS, basing it on the current limit value provides a more reliable design since the circuit can operate near the current limit threshold continuously. The resistor’s surge capability must also be considered since the circuit breaker threshold is approximately twice the current limit threshold. Connections from RS to the LM25069 should be made using Kelvin techniques. In the suggested layout of Figure 7 the small pads at the lower corners of the sense resistor connect only to the sense resistor terminals, and not to the traces carrying the high current. With this technique, only the voltage across the sense resistor is applied to VIN and SENSE, eliminating the voltage drop across the high current solder connections. 30086719 FIGURE 7. Sense Resistor Connections POWER LIMIT THRESHOLD The LM25069 determines the power dissipation in the external MOSFET (Q1) by monitoring the drain current (the current in RS), and the VDS of Q1 (SENSE to OUT pins). The resistor at the PWR pin (RPWR) sets the maximum power dissipation for Q1, and is calculated from the following equation: RPWR = 2.32 x 105 x RS x PFET(LIM) (2) where PFET(LIM) is the desired power limit threshold for Q1, and RS is the current sense resistor described in the Current Limit section. For example, if RS is 10 mΩ , and the desired power limit threshold is 20W, RPWR calculates to 46.4 kΩ. If Q1’s power dissipation reaches the threshold Q1’s gate is modulated to regulate the load current, keeping Q1’s power from exceeding the threshold. For proper operation of the power limiting feature, RPWR must be ≤150 kΩ. While the power limiting circuit is active, the fault timer is active as described in the Fault Timer & Restart section. Typically, power limit is reached during startup, or if the output voltage falls due to a severe overload or short circuit. The programmed maximum power dissipation should have a reasonable margin from the maximum power defined by the FET's SOA chart if the LM25069-2 is used since the FET will be repeatedly stressed during fault restart cycles. The FET manufacturer should be consulted for guidelines. If the application does not require use of the power limit function the PWR pin can be left open. The accuracy of the power limit function at turn-on may degrade if a very low value power dissipation limit is set. The reason for this caution is that the voltage across the sense resistor, which is monitored and regulated by the power limit circuit, is lowest at turn-on when the regulated current is at minimum. The voltage across the sense resistor during power limit can be expressed as follows: where IL is the current in RS, and VDS is the voltage across Q1. For example, if the power limit is set at 20W with RS = 10 mohms, and VDS = 15V the sense resistor voltage calculates to 13.3 mV, which is comfortably regulated by the LM25069. However, if a lower power limit is set lower (e.g., 2W), the sense resistor voltage calculates to 1.33 mV. At this low level noise and offsets within the LM25069 may degrade the power limit accuracy. To maintain accuracy, the sense resistor voltage should not be less than 5 mV. TURN-ON TIME The output turn-on time depends on whether the LM25069 operates in current limit, or in both power limit and current limit, during turn-on. www.national.com 14 LM25069 A) Turn-on with current limit only: The current limit threshold (ILIM) is determined by the current sense resistor (RS). If the current limit threshold is less than the current defined by the power limit threshold at maximum VDS the circuit operates at the current limit threshold only during turn-on. Referring to Figure 10a, as the load current reaches ILIM, the gate-tosource voltage is controlled at VGSL to maintain the current at ILIM. As the output voltage reaches its final value, (VDS ≊ 0V) the drain current reduces to its normal operating value. The time for the OUT pin voltage to transition from zero volts to VSYS is equal to: 30086723 FIGURE 9. Load Draws Current During Turn-On where CL is the load capacitance. For example, if VSYS = 12V, CL = 1000 µF, and ILIM = 1A, tON calculates to 12 ms. The maximum instantaneous power dissipated in the MOSFET is 12W. This calculation assumes the time from t1 to t2 in Figure 10a is small compared to tON, and the load does not draw any current until after the output voltage has reached its final value, and PGD switches high (Figure 8). If the load draws current during the turn-on sequence (Figure 9), the turn-on time is longer than the above calculation, and is approximately equal to: B) Turn-on with power limit and current limit: The maximum allowed power dissipation in Q1 (PFET(LIM)) is defined by the resistor at the PWR pin, and the current sense resistor RS. See the Power Limit Threshold section. If the current limit threshold (ILIM) is higher than the current defined by the power limit threshold at maximum VDS (PFET(LIM)/VSYS) the circuit operates initially in the power limit mode when the VDS of Q1 is high, and then transitions to current limit mode as the current increases to ILIM and VDS decreases. See Figure 10b. Assuming the load (RL) is not connected during turn-on, the time for the output voltage to reach its final value is approximately equal to: where RL is the load resistance. The Fault Timeout Period must be set longer than tON to prevent a fault shutdown before the turn-on sequence is complete. For example, if VSYS = 12V, CL = 1000 µF, ILIM = 1A, and PFET(LIM) = 10W, tON calculates to ≊12.2 ms, and the initial current level (IP) is approximately 0.83A. The Fault Timeout Period must be set longer than tON. 30086722 FIGURE 8. No Load Current During Turn-On 15 www.national.com LM25069 30086725 FIGURE 10. MOSFET Power Up Waveforms MOSFET SELECTION It is recommended that the external MOSFET (Q1) selection be based on the following criteria: - The BVDSS rating should be greater than the maximum system voltage (VSYS), plus ringing and transients which can occur at VSYS when the circuit card, or adjacent cards, are inserted or removed. - The maximum continuous current rating should be based on the current limit threshold (50 mV/RS), not the maximum load current, since the circuit can operate near the current limit threshold continuously. - The Pulsed Drain Current spec (IDM) must be greater than the current threshold for the circuit breaker function (95 mV/ RS). - The SOA (Safe Operating Area) chart of the device, and the thermal properties, should be used to determine the maximum power dissipation threshold set by the RPWR resistor. The programmed maximum power dissipation should have a reasonable margin from the maximum power defined by the FET's SOA chart if the LM25069-2 is used since the FET will be repeatedly stressed during fault restart cycles. The FET manufacturer should be consulted for guidelines. - RDS(on) should be sufficiently low that the power dissipation at maximum load current (IL(max)2 x RDS(on)) does not raise its junction temperature above the manufacturer’s recommendation. If the circuit’s input voltage is at the low end of the LM25069’s operating range (14V), the gate-to-source voltage applied to the MOSFET by the LM25069 is less than 5V, and can approach 1V in a worst case situation. See the graph “GATE Pin Voltage”. The selected device must have a suitable Gate-to-Source Threshold Voltage. The gate-to-source voltage provided by the LM25069 can be as high as 19.5V at turn-on when the output voltage is zero. At turn-off the reverse gate-to-source voltage will be equal to the output voltage at the instant the GATE pin is pulled low. If the device chosen for Q1 is not rated for these voltages, an external zener diode must be added from its gate to source, with the zener voltage less than the device maximum VGS rating. The zener diode’s working voltage protects the MOSFET during turn-on, and its forward voltage protects the MOSFET during shutoff. The zener diode’s forward current rating must be at least 260 mA to conduct the GATE pull-down current when a circuit breaker condition is detected. TIMER CAPACITOR, CT The TIMER pin capacitor (CT) sets the timing for the insertion time delay, fault timeout period, and restart timing of the LM25069-2. A) Insertion Delay - Upon applying the system voltage (VSYS) to the circuit, the external MOSFET (Q1) is held off during the insertion time (t1 in Figure 3) to allow ringing and transients at VSYS to settle. Since each backplane’s response to a circuit card plug-in is unique, the worst case settling time must be determined for each application. The insertion time starts when VIN reaches the POR threshold, at which time the internal 5.5 µA current source charges CT from 0V to 1.72V. The required capacitor value is calculated from: For example, if the desired insertion delay is 250 ms, CT calculates to 0.8 µF. At the end of the insertion delay, CT is quickly discharged by a 2 mA current sink. B) Fault Timeout Period - During in-rush current limiting or upon detection of a fault condition where the current limit and/ or power limit circuits regulate the current through Q1, the fault timer current source (80 µA) switches on to charge CT. The Fault Timeout Period is the time required for the voltage at the TIMER pin to transition from ground to 1.72V, at which time Q1 is switched off. If the LM25069-1 is in use, the required capacitor value is calculated from: (3) For example, if the desired Fault Timeout Period is 17 ms, CT calculates to 0.8 µF. When the Fault Timeout Period expires, the LM25069-1 latches the GATE pin low until a powerup sequence is initiated by external circuitry. If the LM25069-2 is in use, the Fault Timeout Period during restart cycles is approximately 18% shorter than the initial fault timeout period which initiated the restart cycles since the voltage at the TIMER pin transitions from 0.3V to 1.72V. Since the Fault Timeout Period must always be longer than the turn-on-time, www.national.com 16 LM25069 the required capacitor value for the LM25069-2 is calculated using this shorter time period: (4) For example, if the desired Fault Timeout Period is 17 ms, CT calculates to 0.96 µF. When the Fault Timeout Period of the LM25069-2 expires, a restart sequence starts as described below (Restart Timiing). Since the LM25069 normally operates in power limit and/or current limit during a power-up sequence, the Fault Timeout Period MUST be longer than the time required for the output voltage to reach its final value. See the Turn-on Time section C) Restart Timing For the LM25069-2, after the Fault Timeout Period described above, CT is discharged by the 2.5 µA current sink to 1.0V. The TIMER pin then cycles through seven additional charge/discharge cycles between 1V and 1.72V as shown in Figure 5. The restart time ends when the TIMER pin voltage reaches 0.3V during the final high-to-low ramp. The restart time, after the Fault Timeout Period, is equal to: - The lower OVLO threshold (VOVL) cannot be chosen in advance in this case, but is determined after the values for R1-R3 are determined. If VOVL must be accurately defined in addition to the other three thresholds, see Option B below. The resistors are calculated as follows: The lower OVLO threshold is calculated from: As an example, assume the application requires the following thresholds: VUVH = 8V, VUVL = 7V, VOVH = 15V. = CT x 2.65 x 106 For example, if CT = 0.8 µF, tRESTART = 2.12 seconds. At the end of the restart time, Q1 is switched on. If the fault is still present, the fault timeout and restart sequence repeats. The on-time duty cycle of Q1 is approximately 0.67% in this mode. UVLO, OVLO By programming the UVLO and OVLO thresholds the LM25069 enables the series pass device (Q1) when the input supply voltage (VSYS) is within the desired operational range. If VSYS is below the UVLO threshold, or above the OVLO threshold, Q1 is switched off, denying power to the load. Hysteresis is provided for each threshold. Option A: The configuration shown in Figure 11 requires three resistors (R1-R3) to set the thresholds. The lower OVLO threshold calculates to 13.9V, and the OVLO hysteresis is 1.1V. Note that the OVLO hysteresis is always slightly greater than the UVLO hysteresis in this configuration. When the R1-R3 resistor values are known, the threshold voltages and hysteresis are calculated from the following: VUV(HYS) = R1 x 20 µA 30086729 FIGURE 11. UVLO and OVLO Thresholds Set By R1-R3 The procedure to calculate the resistor values is as follows: - Choose the upper UVLO threshold (VUVH), and the lower UVLO threshold (VUVL). - Choose the upper OVLO threshold (VOVH). VOV(HYS) = (R1 + R2) x 20 µA Option B: If all four thresholds must be accurately defined, the configuration in Figure 12 can be used. 17 www.national.com LM25069 As an example, assume the application requires the following thresholds: VUVH = 8V, VUVL = 7V, VOVH = 15.5V, and VOVL = 14V. Therefore VUV(HYS) = 1V, and VOV(HYS) = 1.5V. The resistor values are: R1 = 50 kΩ, R2 = 10 kΩ R3 = 75 kΩ, R4 = 6.07 kΩ Where the R1-R4 resistor values are known, the threshold voltages and hysteresis are calculated from the following: 30086741 FIGURE 12. Programming the Four Thresholds The four resistor values are calculated as follows: - Choose the upper and lower UVLO thresholds (VUVH) and (VUVL). VUV(HYS) = R1 x 20 µA VOV(HYS) = R3 x 20 µA Option C: The minimum UVLO level is obtained by connecting the UVLO pin to VIN as shown in Figure 13. Q1 is switched on when the VIN voltage reaches the POR threshold (≊2.6V). The OVLO thresholds are set using R3, R4. Their values are calculated using the procedure in Option B. -Choose the upper and lower OVLO threshold (VOVH) and (VOVL). 30086750 FIGURE 13. UVLO = POR with Shutdown/Restart Control Option D: The OVLO function can be disabled by grounding the OVLO pin. The UVLO thresholds are set as described in Option B or Option C. POWER GOOD PIN During turn-on, the Power Good pin (PGD) is high until the voltage at VIN increases above ≊ 1.6V. PGD then switches low, remaining low as the VIN voltage increases. When the voltage at OUT increases to within 1.3V of the SENSE pin (VDS